Positive Muon Decay

Cartoon of positive muon (µ+) decay showing the case where the neutrino and antineutrino go off in the same direction together, which gives the highest energy positron (e+), roughly 52 MeV. Since (in this case) the spins of the neutrino and antineutrino cancel each other, the positron spin must be in the same direction as that of the muon in order to balance angular momentum. In the weak interaction governing this decay, such an ultrarelativistic positron acts pretty much like an antineutrino and therefore can only be created with positive helicity (right-handed, with its spin pointing along its momentum). Therefore the reaction has maximum probability when the positron exits along the direction of the muon's spin (and zero probability of being emitted in the opposite direction).

When averaged over all possible emission directions and energies of the neutrino and antineutrino, the decay asymmetry or anisotropy has a value of 1/3 (the positron is 2/3 more likely to be emitted along the muon spin than opposite to it). So an ensemble of polarized positive muons ``broadcasts'' its polarization in a shower of high energy positrons that will penetrate through the walls of cryostats, scintillators and other experimental apparatus. This makes µSR possible.
Jess H. Brewer
Last modified: Fri Dec 5 09:50:44 EST